Coprecipitation of neptunium(IV) and plutonium(IV) with hydrolyzed iron(III) in carbonate solutions

In regeneration of Np(IV)-and Pu(IV)-containing recycled solvent by treatment with aqueous sodium carbonate contaminated with iron, some portion of Np(IV) and Pu(IV) coprecipitates with hydrolyzed iron. The degree of coprecipitation of Np and Pu depends on both the iron and sodium carbonate concentrations. The presence of n-dibutyl hydrogen phosphate in the recycled solvent before its regeneration does not noticeably affect the coprecipitation of Np and Pu. The possible mechanisms of coprecipitation of actinides with hydrolyzed iron in carbonate solutions are discussed.     

Features of Pu(IV) and Np(IV) Extraction from Nitric Acid Solutions of Uranyl Nitrate with 30% TBP

The extraction of Pu(IV) and Np(IV) from nitric acid solutions containing high concentrations of uranyl nitrate with 30% TBP in hydrocarbon diluent was studied. It was found that, as the Pu(IV) and Np(IV) concentration grows from tens milligrams to several grams at fixed uranyl nitrate (100 g l^-1 and higher) and nitric acid concentrations in the aqueous phase, the distribution coefficients of actinides(IV) increase (for Np to a greater extent than for Pu). This trend becomes more pronounced at higher temperatures. Correlation equations describing this effect are suggested.     

Reduction of Pu(IV) and Np(VI) with carbohydrazide in nitric acid solution

The reduction of Pu(IV) and Np(VI) with carbohydrazide (NH₂NH)₂CO in 1–6 M HNO₃ solutions was studied. The Pu(IV) reduction is described by a first-order rate equation with respect to Pu(IV). At [HNO₃] ≥ 3 M, the reaction becomes reversible. The rate constants of the forward and reverse reactions were determined, and their activation energies were estimated. Neptunium(VI) is reduced to Np(V) at a high rate, whereas the subsequent reduction of Np(V) to Np(IV) is considerably slower and is catalyzed by Fe and Tc ions. The possibility of using carbohydrazide for stabilizing desired combinations of Pu and Np valence states was examined.     

Reduction of Pu(IV) and Np(VI) with hydroxylamine in solutions of low acidity with high uranium content

Decomposition of hydroxylamine in HNO₃ solutions containing 350 to 920 g l^?1 U(VI) was studied. In the absence of fission and corrosion products (Zr, Pd, Tc, Mo, Fe, etc.), hydroxylamine is stable for no less than 6 h at [HNO₃] < 1 M and 60°C. In the presence of these products, the stability of hydroxylamine appreciably decreases. The reduction of Pu(IV) and Np(VI) with hydroxylamine in aqueous 0.33 and 0.5 M HNO₃ solutions containing 850 g l^?1 U(VI) and fission and corrosion products at 60°C was studied. Np(VI) is rapidly reduced to Np(V), after which Np(V) is partially reduced to Np(IV). The rate of the latter reaction in such solutions is considerably higher than the rate of the Np(V) reduction with hydroxylamine in HNO₃ solutions without U(VI). At [HNO₃] = 0.33 M, the use of hydroxylamine results in the conversion of Pu to Pu(III) and of Np to a Np(IV,V) mixture, whereas at [HNO₃] = 0.5 M the final products are Pu(IV) and Np(V).     

Reaction of ozone with Np(IV) and Pu(IV) oxalates in water

The reaction of the ozone–oxygen mixture with aqueous suspensions of Np(IV) and Pu(IV) oxalates was studied. Both metal cations and oxalate anions are oxidized in the process. The final products are Np(VI) and Pu(VI) hydroxides. The composition of Np(VI) hydroxide was confirmed by X-ray diffraction analysis. Oxidation of Np(IV) oxalate with oxygen leads to the accumulation of Np(V) oxalate and oxalic acid in the solution. At incomplete oxidation of Np(IV) oxalate with ozone in water, Np(V) is also accumulated. Heating considerably accelerates the ozonation. The possible reaction mechanism is briefly discussed. The Np(V) and Np(VI) ions participate in the catalytic cycle of the decomposition of oxalate ions with ozone.     

Behavior of Cs, Np(V), Pu(IV), and U(VI) in pore water of bentonite

Sorption of Cs, Pu(IV), Np(V), and U(VI) with bentonite from solutions was studied. Physicochemical species of radionuclides in the liquid phase were determined, the sorption mechanisms were established, and the influence of bentonite colloids on the behavior of radionuclides was studied. It was shown that Cs is sorbed by the ion-exchange mechanism, whereas the sorption of actinides at pH > 5 is governed by the reaction with surface hydroxy groups of betonite, and at pH < 5 the competing processes are ion exchange and complex formation. Reduction of Np(V) and U(VI) to Np(IV) and U(IV) in the solution with Fe(II) compounds present in the system was proved by the extraction method. Various methods of separating the solid phase were used in studying the dependence of the distribution coefficients of Np and Pu on the ratio of pore water and bentonite; it was shown that Np and Pu are sorbed on bentonite colloids.     

Oxidation of Np(IV) with hydrogen peroxide in carbonate solutions

Oxidation of Np(IV) with hydrogen peroxide in NaHCO₃-Na₂CO₃ solutions was studied by spectrophotometry. In NaHCO₃ solution, Np(IV) is oxidized to Np(V) and partially to Np(VI). It follows from the electronic absorption spectra that Np(IV) in 1 M Na₂CO₃ forms with H₂O₂ a mixed peroxide-carbonate complex. Its stability constant β is estimated at 25–30. The Np(IV) bound in the mixed complex disappears in a first-order reaction with respect to [Np(IV)]. The first-order rate constant k’ is proportional to [H₂O₂] in the H₂O₂ concentration range 2.5–11 mM, but further increase in [H₂O₂] leads to a decrease in k′. The bimolecular rate constant k = k′/[H₂O₂] in solutions containing up to 11 mM H₂O₂ increases in going from 1 M NaHCO₃ to 1 M Na₂CO₃ and significantly decreases with a further increase in the carbonate content. The activated complex is formed from Np(IV) peroxide-carbonate and carbonate complexes. Synchronous or successive electron transfer leads to the oxidation of Np(IV) to Np(V). Large excess of H₂O₂ oxidizes Np(V) to Np(VI), which is then slowly reduced. As a result, Np(V) is formed in carbonate solutions at any Np(IV) and H₂O₂ concentrations.     

Effect of hydrogen peroxide on precipitation of Pu(IV) in alkaline solutions

Formation of peroxy compounds of Pu(IV) is possible in concentrated alkali solutions despite instability of hydrogen peroxide under these conditions. The resulting peroxy compound of Pu(IV) is fairly stable in alkaline solutions. The [Pu]: [O₂⁻] ratio in the compound is close to 1: 2. The formation of the peroxy compound favors a decrease in the content of colloidal polymeric Pu(IV) species in solution, thus making the Pu precipitation from solution more complete.     

Solubility of mixed-valence U(IV–VI) and Np(IV–V) hydroxides in simulated groundwater and 0.1 M NaClO4 solutions

The kinetics of U(VI) accumulation in the phase of U(IV) hydroxide and of Np(V) in the phase of neptunium(IV) hydroxide, and also the solubility of the formed mixed-valence U(IV)-U(IV) and Np(IV)-Np(V) hydroxides in simulated groundwater (SGW, pH 8.5) and 0.1 M NaClO₄ (pH 6.9) solutions was studied. It was found that the structure of the mixed U(IV–VI) hydroxide obtained by both oxidation of U(IV) hydroxide with atmospheric oxygen and alkaline precipitation from aqueous solution containing simultaneously U(IV) and U(VI) did not affect its solubility at the U(VI) content in the system exceeding 16%. The solubility of mixed-valence U(IV–VI) hydroxides in SGW and 0.1 M NaClO₄ is (3.6±1.9) × 10^?4 and (4.3 ± 1.7) × 10^?4 M, respectively. The mixed Np(IV–V) hydroxide containing from 8 to 90% Np(V) has a peculiar structure controlling its properties. The solubility of the mixed-valence Np(IV–V) hydroxide in SGW [(6.5 ± 1.5) × 10^?6 M] and 0.1 M NaClO₄ [(6.1±2.4) × 10^?6 M] is virtually equal. Its solubility is about three orders of magnitude as high as that of pure Np(OH)₄ (10^?9–10^?8 M), but considerably smaller than that of NpO₂(OH) (～7 × 10^?4 M). The solubility is independent of the preparation procedure [oxidation of Np(OH)₄ with atmospheric oxygen or precipitation from Np(IV) + Np(V) solutions]. The solubility of the mixed-valence Np hydroxide does not increase and even somewhat decreases [to (1.4±0.7) × 10^?6 M] in the course of prolonged storage (for more than a year).     

Reaction of ozone with Np(V) and Np(IV) in carbonate solutions

A spectrophotometric study showed that ozone in concentrated carbonate solutions forms complexes with CO ₃ ^2? ions, which inhibits the ozone decomposition. Free ozone oxidizes Np(V) at high rate. The bound ozone reacts with Np(V) at moderate rate. Np(IV) reacts with O₃ slowly, with Np(VI) formed in NaHCO₃ solution and only Np(V) formed in Na₂CO₃ solution.     

Ruthenium-catalyzed oxidation of Np(IV) in nitric acid solutions

Oxidation of Np(IV) with nitric acid in the presence of Ru/SiO₂ catalysts was studied by spectrophotometry. The catalytic oxidation of Np(IV) in nitric acid solutions occurs even in the presence of hydrazine. The mechanism of Ru-catalyzed oxidation of Np(IV) with nitric acid was suggested on the basis of the kinetic data. The effect of the Ru nanoparticle size on the activation energy of the catalytic oxidation of Np(IV) was revealed. It shows that the heterogeneous-catalytic reaction is structure-sensitive (positive size effect).     

Redox Kinetics of U,Pu, and Np in TBP Solutions: VI. Reactions of Neptunium and Plutonium with Organic Reducing Agents: Determination of the Final Oxidation States and Reaction Rates

Reactions of Pu(IV) and Np(VI) with organic reducing agents of various types (substituted hydroxylamines, oximes, aldehydes, etc.) in tributyl phosphate solutions containing nitric acid were studied spectrophotometrically. The molar extinction coefficients of neptunium and plutonium in various oxidation states [Np(IV,V,VI), Pu(III,IV,VI)] in TBP solutions were determined as influenced by HNO₃ and H₂O concentrations and temperature. It was found that organic reducing agents at low HNO₃ concentration convert plutonium and neptunium to Pu(III) and Np(V), respectively. With increasing HNO₃ concentration Pu(III) and Np(V) are partly oxidized back to Pu(IV) and Np(VI), respectively, by reaction with nitrous acid. The rate constants of Pu(VI) and Np(VI) reduction and Np(V) oxidation as influenced by concentration of organic reducing agents and HNO₃ were evaluted from the kinetic data.     

Reduction of Np(VI) and Pu(VI) with anions of some substituted carboxylic acids

The reaction of Np(VI) with organic acid anions in solutions containing lithium salts of tartaric, malic, α-aminoglutaric, and trihydroxyglutaric acids was studied. Changes in the solution spectra show that Np(VI) forms complexes with organic acid anions, which is followed by the reduction of Np(VI) to Np(V). Similar processes occur in solutions containing Pu(VI) and sodium phenylglycolate or ammonium salicylate. In weakly acidic solutions, the loss of the Np(VI) and Pu(VI) concentrations is a linear function of time. The possible mechanism of the redox reactions was suggested.     

Disproportionation of polymeric Pu(IV) in weakly acidic solutions

Polymeric Pu(IV) in aqueous solutions in the pH range 0.5–3 disproportionates with time to form Pu(III) and Pu(VI). The arising Pu(III) is bound by hydroxyl groups of polymeric Pu(IV) and does not exhibit intrinsic absorption bands in the spectrum of a solution of polymeric Pu(IV). However, after ultrafiltration of the solution through a filter with a pore size of ∼3 nm Pu(III) is clearly identified in the filtrate by its absorption maxima. Pu(VI) occurs in the solution in the ionic state and is not bound by hydroxy groups of polymeric Pu (IV). Therefore, Pu(VI) is identified in the solution absorption spectrum both before ultrafiltration and after it. Thus, storage of solutions of polymeric Pu(IV) with pH 0.5–3 is accompanied by formation of Pu(III) and Pu(VI) ions.     

Formation of polymeric Pu(IV) hydroxide structures in aqueous solutions

Forms of occurrence of polymeric Pu(IV) in simulated groundwater (SGW) were studied spectrophotometrically and by the method of centrifugal ultrafiltration through filtering inserts permeable to polymeric Pu species with different molecular weights. The dependences of the fractions of definite Pu(IV) forms on the total Pu content in the solution were found. The possibility of formation of Pu(IV) quasipolymeric structures in aqueous solutions was considered in relation to the problem of the transfer of radioactive contaminants with underground water. Equilibrium distribution of Pu(IV) polymers depending on the total Pu(IV) concentration in the solution was analyzed theoretically. From the experimental data obtained, the parameter allowing determination of the weight distribution of the polymeric particles in relation to the total Pu(IV) concentration was theoretically calculated, and their equilibrium distributions depending on the total Pu(IV) concentration were found.     

Kinetics of Redox Reactions of U, Pu, and Np in TBP Solutions: VII. Kinetics of Reduction of Pu(IV) and Np(VI) with Butanal Oxime in Undiluted TBP

The kinetics of reduction of Pu(IV) and Np(VI) with butanal oxime in undiluted TBP containing HNO₃ was studied spectrophotometrically. In the range [HNO₃] = 0.08-0.75 M the rate of Pu(IV) reduction is described by the equation -d[Pu(IV)]/dt = k[Pu(IV)]²[C₃H₇CHNOH]/{[Pu(III)][HNO₃]²} with the rate constant k = 0.068±0.017 mol l^-1 min^-1 at 20°C. The kinetic equation of the reduction of Np(VI) to Np(V) in the range [HNO₃] = 0.01-0.27 M is -d[Np(VI)]/dt = k[Np(VI)][C₃H₇CHNOH][H₂O]²/[HNO₃]0.5, where k = 0.058±0.007 l2.5 mol-2.5 min^-1 at 25°C, and the activation energy is 79±9 kJ mol^-1.     

Factors governing the Pu(IV) speciation in solutions

After storage of Pu(IV) hydroxide for more than 4 months, ～90% of this compound polymerizes, the remainder (～ 10%) being weakly polymerized Pu(IV). In 0.01 M NaCl solutions (pH ～4–10) being in equilibrium with mixed or polymeric Pu(OH)₄ (decantates), plutonium is mainly in the form of highly polymerized colloidal particles of molecular weight exceeding 100 kDa. Therefore, the Pu concentration in the solutions prepared by decantation or centrifugation of decanted solutions can range from 10^?4 to 10^?7 M. The content of weakly polymerized Pu in solutions varies from 10^?7 to 10^?9 M and depends on pH of the solution in the range 4–6. This dependence is virtually absent at pH 6–10.     

Reaction of Np(V) with U(IV) in solutions with pH > 2

Oxidation of U(IV) with Np(V) in bicarbonate-carbonic acid solutions and the nature and reactivity of actinide(IV) compounds formed in these media were studied spectrophotometrically.     

Complexation of tetravalent actinides (Th, U, Np, Pu) with 2,6-pyridinedicarboxylic acid in aqueous solutions

The interaction of An(IV) ions (An = Th, U, Np, Pu) with 2,6-pyridinedicarboxylic acid (2,6-PDCA) in solutions was studied by spectrophotometry. The electronic absorption spectra of the individual complex species An(PDC)²⁺, An(PDC)₂, and An(PDC) ₃ ^2? (PDC^2? is 2,6-PDCA anion; An = U, Np, Pu) were obtained. At [2,6-PDCA] ? 3[An(IV)] + 0.01 M and [H⁺] ? 0.2 M, the prevalent An(IV) species are the complexes An(PDC) ₃ ^2? . Their overall stability constant exceeds 10²⁵ L³ mol^?3 and increases in the series from Th(IV) to Pu(IV) by ～8 orders of magnitude. Very high stability of An(IV) complexes with 2,6-PDCA anions leads to significant shifts of the redox potentials of couples involving An(IV). In particular, large difference in the stability of An(III) and An(IV) complexes is responsible for the fact that Pu(III) in the presence of 2,6-PDCA is readily oxidized with atmospheric oxygen to Pu(IV).     

Mechanism of transformation of Np(V) into Np(IV) in sodium 1,2-cyclohexanediaminetetraacetate solutions

The kinetics of the transformation of Np(V) into Np(IV) in 0.1 M potassium biphthalate solutions containing 5–74 mM sodium 1,2-cyclohexanediaminetetraacetate (Na₂CHDTA) or in a 96–97 mM Na₂CHDTA solution at 25–45°С was studied. The reaction rate at Na₂CHDTA concentrations in the range 5–60 mM and pH 3.5–5.9 is described by the equation V = k[Np(V)]^1.4[CHDTA], and at Na₂CHDTA concentrations in the range 70–100 mM and pH 4.1–5.2, by the equation V = k _A[Np(V)]^1.4. Neptunium(V) forms with the CHDTA ion an activated complex in which Np(V) is reduced to Np(IV). The dimer {Np(V)}₂ forming another activated complex with the CHDTA ion is formed concurrently. The latter complex decomposes along the disproportionation pathway to give Np(IV) and Np(VI). Np(VI) is reduced with the CHDTA ion to Np(V).